Field of the Invention
The present invention relates to a compound semiconductor substrate.
Description of the Related Art
When manufacturing a compound semiconductor substrate by stacking compound semiconductor layers on a ground substrate, a ground substrate having a crystal plane according to the crystal orientation of the compound semiconductor layer to be formed on one main surface is used, and examples of the ground substrate may include a silicon (Si) single crystal substrate or a sapphire single crystal substrate.
In recent years, there has been an increasing need for the compound semiconductor substrate having a larger diameter or a thicker compound semiconductor layer. However, warpage is caused when compound semiconductors are stacked on a single crystal substrate when the single crystal substrate has a larger diameter. In addition, the single crystal substrate is required to be processed in consideration of the plane orientation or toughness due to its quality of material so that the design thereof is limited. Furthermore, the cost of the single crystal substrate itself, particularly a sapphire substrate or a SiC substrate is high. Hence, it is difficult to sufficiently satisfy these requirements as long as a single crystal substrate is applied as the ground substrate.
Accordingly, it is investigated again to use a sintered body material that is easily increased in diameter and relatively inexpensive in price as a ground substrate.
JP 2006-315951 A discloses a technology that there is a thin film substrate obtained by forming a thin film composed mainly of at least one kind selected from gallium nitride, indium nitride, and aluminum nitride on a sintered body composed mainly of a ceramic material, in which the thin film is obtained by forming a single crystal thin film on at least any one selected from an amorphous thin film, a polycrystalline thin film, and an oriented polycrystalline thin film, or in which the thin film is obtained by forming a single crystal thin film on a single crystal thin film and the crystallinity of the single crystal thin film formed on the other single crystal thin film is equal or superior to the crystallinity of the other single crystal thin film.
WO 2012/043474 A1 discloses a polycrystalline aluminum nitride substrate for GaN-based semiconductor crystal growth which contains a sintering aid component at from 1 to 10 mass %, has a thermal conductivity of 150 W/m·K or more, and does not have a recess having a maximum diameter greater than 200 μm on the substrate surface as a substrate material for the grain growth of a GaN-based semiconductor.
JP 2007-112633 A discloses a nitride semiconductor wafer including a substrate that is composed of polycrystalline aluminum nitride having orientation and has a plurality of steps formed on the main surface and a single crystal nitride semiconductor layer formed on the main surface of the substrate and a nitride semiconductor element including an electrode formed on the nitride semiconductor layer for the purpose of providing a nitride semiconductor wafer and a nitride semiconductor element which are excellent in crystallinity while using a polycrystalline AlN substrate.
JP 2013-258373 A discloses a method for manufacturing a composite substrate which includes a step of preparing a sintered substrate having an average particle size of 0.1 μm or more and 30 μm or less, a step of bonding the sintered substrate with a semiconductor crystal substrate by interposing a bonding layer, and a step of forming a composite substrate in which a semiconductor crystal layer is bonded on the sintered substrate by interposing the bonding layer by separating a part of the semiconductor crystal layer from the semiconductor crystal substrate.
JP 5755390 B1 and JP 5756888 B2 disclose a technology in which a composite substrate obtained by bonding a single crystal silicon donor substrate on a handle substrate of a polycrystalline material can be applied to a power device using a compound semiconductor.
It is advantageous in terms of cost if it is possible to form a single crystal layer directly on a polycrystalline surface, but it is practically difficult to form a highly crystalline semiconductor layer on a polycrystalline surface by the technologies described in JP 2006-315951 A and WO 2012/043474 A1.
In addition, for example, in JP 2007-112633 A, it is disclosed that the c-axis is approximately parallel to the growth direction and further the a-axis or m-axis is approximately parallel or perpendicular to the extending direction of the step, and a single crystal nitride semiconductor layer can be grown on a polycrystalline AlN substrate when a plurality of steps having a height, for example, of about several nanometers are formed on the surface of a polycrystalline AlN substrate so as to be substantially parallel to each other and a nitride semiconductor is grown on this so that a single crystal layer can be formed directly on a polycrystalline surface.
However, it is greatly costly to uniformly and equally form nano-size steps over the entire one main surface of a large-size substrate and it is concerned that the steps are not uniform due to the presence of grain boundaries and voids on the polycrystalline substrate surface.
In the method as described in JP 2013-258373 A in which a semiconductor crystal layer is prepared in advance and bonded with a polycrystalline substrate via a bonding layer and a part of the semiconductor crystal layer is separated, a considerable thickness is required in the bonding step or the separation step, but the manufacturing cost increases when the material for the semiconductor crystal layer to be prepared in advance is expensive. Furthermore, in this case, it is difficult to fabricate a stacked structure of semiconductor crystal layers having different compositions and the degree of freedom in design as a semiconductor device is limited.
The inventions described in JP 5755390 B1 and JP 5756888 B2 relate to a composite substrate obtained by bonding a single crystal body on one main surface of a polycrystalline body. It is also attempted to fabricate a variety of compound semiconductor substrates by stacking a compound semiconductor layer on a composite substrate by the same method for stacking a compound semiconductor layer on a single crystal substrate as in the prior art.
Meanwhile, an excellent effect to be equal to or higher than that in the case of stacking a compound semiconductor layer on a single crystal substrate is expected in the case of applying the structure of a compound semiconductor layer of the prior art on the composite substrate. However, it is not sufficiently clarified which structure of a compound semiconductor and a composite substrate is a preferred form.